Microphones are transducers; assemblies that convert one form of energy into another. In the case of microphones, they convert sound waves—periodic displacements of pressure in air—into electrical impulses. These impulses are then used in electronic reproduction of the original sound. Microphones operate utilizing a diaphragm, typically a flat disc that reacts to pressure changes in the air. The sound to be reproduced creates periodic waves in the air, which displaces the diaphragm from its resting place. The diaphragm, housed in what is commonly referred to as a capsule, acts as the first stage of the transducer, converting physical air pressure changes in the form of sound waves, into electrical impulses via a variety of methods.
Most modern vocal recording is done with the singer or announcer within six inches or less of the microphone. This often creates unwanted artifacts that occur when plosives (e.g., hard consonants such as p's, b's) from the singer or announcer's mouth result in bursts of air that, when they reach the diaphragm, cause it to travel in such a way that creates distortion of the desired sound. This is manifested to the listener as a low frequency pop or hump sound emanating from the electronic reproduction audio system. This is considered disruptive to the listener, as it is not an element that would be present acoustically if the listener was within physical proximity to the singer or announcer such that they could hear the sounds from the performer's mouth without the aid of an electronic reproduction system.
Conventional attempts to mitigate unwanted artifacts from a singer or announcer rely on the use of pop filters, or windscreens, of various constructions. There are several, often competing considerations in the design and construction of microphone pop filters. First, an apparatus should be as effective as possible in diminishing the negative artifacts of plosive consonants resulting from microphone positioning close to a vocal performer's mouth. Second, an apparatus should cause the fewest anomalies possible in the fidelity of the recording through said microphone. While microphones and electronic reproduction systems themselves do not approach the characteristics of human hearing of the same source in an acoustic environment, one measure of the overall quality of said apparatus is its sonic transparency. Put another way, the frequency response of the receiving microphone should not be altered significantly from that which would occur without the plosive reduction apparatus.
The first consideration can be measured by recording and measuring the amplitude of plosive consonants without the apparatus, then doing the same with the apparatus in place between the audio source (e.g., vocalist or announcer) and the microphone. This may be normally represented as a simple x-y graph, where x is a horizontal axis representing time and y is a vertical axis representing amplitude.
The second consideration—fidelity of frequency response—can be measured by recording and graphically representing the frequency response of a given acoustic source through the microphone and recording apparatus, then doing the same with the plosive reduction apparatus in place between the sound source (vocalist) and the microphone. A comparison of these two resultant graphs provides a measurement of the degree of anomalies introduced by the insertion of the plosive reduction apparatus between the sound source and the microphone. There are several methods of representing this graphically; one in which the sound as received through the transducer (microphone) is an x-y graph where x is the horizontal axis representing the spectrum (in cycles per second, or hertz) of human hearing, and y is the vertical axis representing the amplitude of those frequencies in relation to each other at one moment in time. The second method of graphic representation is a spectrogram, a three dimensional graphic representation of the sound received through the transducer (microphone), where the x axis represents frequency, the y axis represents amplitude, and the z axis represents time. In the case of a plosive filter, the first representation is adequate, as the plosive is typically representative of a relatively short (approximately 5 millisecond) period of time.
Conventional devices incorporate several different methodologies for shielding a microphone diaphragm from the distortion-causing burst of air or wind created from a sung or spoken plosive consonant. One conventional design incorporates a baffle system that is integral to the microphone capsule assembly. In such a construction, a series of physical baffles between the receiving end of the capsule and the diaphragm create a twisting path that acts as a series of barriers, around which the sound wave must travel to reach the diaphragm. In theory, the excess displacement of air resulting in the unwanted distorted plosive is dissipated by the series of baffles, yet the open spaces around the baffles allow the desired normal sound waves to pass through to the capsule and diaphragm.
A second type of design is made of open cell foam. This can be either an integral part of the capsule assembly or an external piece of foam in a variety of shapes with a hollow area into which the microphone is inserted. In theory, the network of foam cells acts as a complex baffle, which prevents the excess displacement of air from a plosive from reaching the microphone diaphragm, yet still allows the desired normal sound waves to pass through to the capsule and diaphragm.
A third type of design is an external hoop and fabric type, consisting of one or more layers of a permeable fabric such as Lycra or spandex that is stretched over a hoop-shaped frame. The fabric is held in place by a system of tightly-fitting concentric hoops, with the fabric edges secured by the pressure between the two hoops. This hoop assembly is attached to the microphone or to a microphone stand by a length of coiled metal whose shape retention allows the user to place the hoop type screen between the mouth of the singer or announcer and the capsule and diaphragm of the microphone. The hoop is affixed to one end of the length of coiled metal, commonly referred to as a “gooseneck.” The other end of this gooseneck incorporates a clip, which is affixed to the microphone body, or the microphone stand. The fabric covered hoop is positioned with the flat face of the hoop assembly facing the vocalist's mouth, so the sound waves resulting in air pressure changes hit the flat surface of the stretched fabric at a 90° angle. In theory, the unwanted excess air movement created by a plosive is reflected and dissipated by the fabric, yet the fabric is permeable to the point that the desired sound waves pass through to the capsule and diaphragm.
Although the hoop type of pop filter or windscreen is commonly accepted to be the most efficient of the three types, providing the greatest amount of plosive artifact reduction while affording the highest fidelity of the sound that does reach the diaphragm, it has been found that this design is moderately effective, at best. Further, if the vocalist is too close (e.g., two inches or less) from the device, or if the device is too close to the microphone capsule, the device loses most of its effectiveness in reducing plosive distortion. One remedy for this—in practical use for some time—is the use of two of these hoop screens in succession, with a small airspace separating them. These are now being commercially manufactured in this dual configuration; however, it has been found that their effectiveness is still not optimal.
Therefore, a significant need continues to exist in the art for an improved device and methodology for attenuating plosive artifacts from an audio source such as a singer or announcer.